TOKYO, June 23 (Reuters) - Nippon Steel & Sumitomo Metal Corp's 5401.T is resuming operations at its Nagoya steel plant in central Japan on Monday after a power outage caused a shutdown the previous day, the third time in less than six months.

The Nogoya plant, which produced 6.74 million tonnes of crude steel in the business year that ended March 31, suffered a power failure on Sunday, leading to a shutdown of the entire plant and release of smoke from coke ovens as a process to burn-off gas accumulated during regular operations.

"One of the four power generators went off after a stop signal, and the other three almost simultaneously halted. We are still investigating the reason," a company spokesman said on Monday.

Nippon Steel, Japan's biggest steelmaker, began gradually restarting some units at the facility on Monday after power supply returned late on Sunday.

All other units, including blast furnaces, are expected to resume operation within a few days, the spokesman said.

He declined to say how much impact the incident will have on its steel output for this year.

No injuries were reported.

A power failure and smoke release occurred at the Nagoya plant twice in January. ID:nL3N0KU1DL

The Nagoya plant, which was built in 1958, is the company's fourth biggest plant in Japan, with its steel sheets being used for automobiles.

"The generator, which triggered the blackout on Sunday, was built about 14 years ago, which is not that old for a generator," the spokesman said.

Nippon Steel, which posted a group recurring profit -- pretax and before one-off items -- of 361 billion yen ($3.54 billion) in the last business year, has said the power failure at the Nagoya plant in January cut its profit by 8 billion yen. ($1 = 102.0500 Japanese Yen)

(Reporting by Yuka Obayashi; Editing by Prateek Chatterjee)

((Yuka.Obayashi@thomsonreuters.com)(+813-6441-1798)(Reuters Messaging: yuka.obayashi.thomsonreuters.com@reuters.net))

Keywords: NIPPON STEEL OUTAGES/